Process for the manufacture of hexafluoropropene



United States Patent US. Cl. 260653.3 8 Claims ABSTRACT OF THE DISCLOSURE Process for preparing hexafiuoropropylene by pyrolyzing tetrafiuoroethylene and/or octafiuorocyclo'butane in the presence of 50 95 mol percent of water vapor under adiabatic conditions at 700 to 900 C. and in which the pyrolysis temperatures are provided by premixing preheated reactants and superheated water vapor before introducing such materials into the pyrolysis reaction zone.

This invention relates to a pyrolysis process for the manufacture of hexafluoropropene from difiuoromethylene homologues such as tetrafiuoroethylene and octafluorocyclobutane.

It is well-known that the pyrolysis of tetrafluoroethylene and its polymers or octafluorocyclobutane at an elevated temperature of about 700-900 C. produces hexafluoropropene. From an industrial view point, however, the methods of pyrolysis previously employed have been found to have disadvantages and to be uneconomical in that the yield or the rate of production of the hexafluoropropene was low or the formation of undesirably byproducts was considerably high. Furthermore, the pyrolsis of tetrafiuoroethylene without the dilution thereof is often dangerous owing to the marked exothermic nature of the hexafluoropropene formation reaction. Therefore, it has been proposed that the pyrolysis be carried out under reduced pressure as low as below 40 mm. Hg. in order to assure the safety of the operation and to improve the hexafluoropropene yield (U.S. 2-758,- 138). A process has also been disclosed which comprises pyrolyzing a mixture of tetrafluoroethylene and chlorodifluoromethane (US. 2,979,539) or higher-boiling fluorocarbon materials (US. 2,970,176). In any case, however, these processes hitherto disclosed have disadvantages which relate either to the inefiiciency caused by the use of reduced pressure therein, or to the formation of by-product hydrogen chloride or gaseous fluorocarbon impurities in large quantities.

It is an object of the present invention to provide an improved pyrolysis method for the production of hexafluoropropene in high yield with a minimum production of undesirably by-products.

It is another object of the present invention to provide an efiicient process for the production of hexafluoropropene which will provide a commercially attractive rate of production together with a satisfactorily safe operation.

The object of the present invention are accomplished by a process which comprises pyrolyzing, at an elevated temperature of about from 700 C. to 900 C. under substantially adiabatic conditions and in the presence of excessive amounts of superheated steam, one species selected from the group consisting of tetrafluoroethylene,

,octafluorocyclobutane and a mixture thereof.

It has been discovered, according to the present invention, that when tetrafluoroethylene, octafluorocyclobutane or a mixture thereof is pyrolyzed at an elevated temperature of about from 700 C. to 900 0, preferably, 800 to 850 C. and a normal pressure in the presence of at least 50 mole percent, and preferably from 60 to 95 mole percent, of superheated steam, a higher yield and a highly efficient production of hexafluoropropene can be effected so as to provide a product of excellent purity and with a high degree of safety at the higher pyrolysis temperatures.

It also has been discovered, according to the present invention, that by pyrolyzing a mixture of tetrafluoroethylene and octafluorocyclobutane having therein an appropriate ratio of the two species in the presence of superheated steam a highly efiicient process for the manufacture of hexafluoropropene is obtained.

The significance of the use of water vapor in the process of the present invention resides not only in the water vapor having a dilution or pressure-reducing effect but also in it having a remarkable effect on the control of the distribution of heat and on the occurance of sidereactions in the reaction phase of the pyrolysis process. As a result of the presence of amounts of superheated steam in the reaction Zone as stipulated by the present invention the formation of undesirable by-products, especially octafiuoroisobutylene, is remarkably suppressed and the pyrolytic conversion of octafiuorocyclobutane is promoted without any accompanying reaction with the water which is present even at an elevated temperature of 850 C.

In the present invention, the reaction is carried out under substantially adiabatic conditions in order to maintain the desired pyrolysis temperature, and such conditions are being obtained by simultaneously introducing into and mixing with the reactant species superheated Water vapor in such amounts and at such temperatures as to insure such conditions, taking into account the initial temperature and mixing ratio of the reactant species.

The simultaneous presence of tetrafluoroethylene and octafluorocyclobutane in the ratios stipulated herein in the reaction phase of the pyrolysis process imparts another remarkable advantage to the process of the present invention in that it provides a synergistic effect on the formation of the hexafluoropropene from both of these reactants which may be considered to have some bearing on the reaction mechanism of the pyrolysis process. Another advantage in the use of the mixture of the two reactants is that the total reaction heat of the formation of hexafluoropropene from both reactants can most conveniently be controlled by counterbalancing the exothermic nature of one pyolysis reaction with the endothermic nature of the other pyrolysis reaction in order that the pyrolysis may be carried out. safely with a high conversion rate at elevated temperatures as high as 850 C. As a result, by pyrolyzing a mixture consisting of tetrafluoroethylene and from 10 to 90 mole percent and preferably from 30 to 60 mole percent, of octafluorocyclobutane at about 850 C., a yield of hexafluoropropene with a conversion of more than 50% of the total of the two reactants may be obtained with a high production rate (space-time-yield) of hexafluoropropene of 30 (mol/l./hr.). The space-time-yield value is determined by the equation Spacetimeyie1d= number of moles of product [Unit volume (liters) of the reactor] [unit time (hours)] Furthermore, when the pyrolysis of the mixture of reactants is carried out, according to the present invention, in the presence of superheated steam in amounts of at least 50 mole percent, preferably, 62 to 68 mole percent of the total gaseous mixture, the synergetic effect is even more pronounced, and the total rate of hexafluoropropene formation as well as the purity of the reaction product is markedly enhanced.

The following examples illustrate specific embodiments of this invention.

Example 1 The experiments for carrying out the pyrolysis process of the present invention were conducted in the following manner with the reaction equipment described below.

Into a cylindrical reactor made of fused silica which is maintained under substantially diabatic conditions within the desired pyrolysis temperature range by proper thermal insulation and external heating in an electrical furnace, the gaseous material to be pyrolyzed, which is preheated to a temperature not exceeding the pyrolytic temperature of the material, and preferably admixed with a suitable diluent gas such as steam or inert gas, is introduced through a narrow first inlet tube. Through another inlet of a jet type fitted in juxtaposition to said first inlet there is introduced under high speed such amounts of superheated steam preheated in a separate heating coil to such temperatures as to provide the entire resulting gas mixture with the required pyrolysis temperature for the reactants mainly by the heat supplied by such steam. In this way the reactant gas and the superheated steam are simultaneously mixed and introduced into the reaction zone of the reactor wherein the gas mixture is heated rapidly to the pyrolysis temperature and pyrolyzed. The pyrolyzed gas is removed rapidly from the reactor through the outlet end of the reactor which is fitted with narrow tubing which leads to a water-scrubber wherein the temperature of the gas mixture is quenched and the mixture is scrubbed with water. Then, the efiluent gas is scrubbed again in a second water-scrubber, dried in a calcium chloride chamber, and condensed in a low-temperature trap cooled with solid carbon dioxide and methanol, the noncondensable gas being collected in a gasometer at ordinary pressure over an aqueous sodium chloride solution. The analysis of the gaseous reaction products was conducted by gas chromatography. Hydrofluoric acid which is liberated from the difluoromethylene material and denoted as dissociated fluorine through elementary decomposition of the material, is obtained as an aqueous solution in the water-scrubbers and the amount thereof is determined by titration with thorium nitrate and alizarin-red.

In Table I there are listed reaction results relating to the pyrolysis of tetrafluoroethylene in a reactor having an interval diameter of 22 mm. and a length of 20.5 mm. in the presence of water vapor at 850:5 C. under a constant space velocity of about 3000 (mole/l./hr.) and at atmospheric pressure with varying amounts of superheated steam. From the data in Table I it may clearly be seen, that by the presence of more than 50 mole percent of water vapor, notwithstanding the large increase of the total space velocity, the conversion rate of tetrafluoroethylene is only slightly affected and the formation of undesirable by-products, especially that of octafiuoroisobutylene, is markedly suppressed.

When tetrafluoroethylene was pyrolyzed in the absence of water conditions which were otherwise the same as those noted above, the reaction temperature was found to be very difiicult and highly inpure reaction products were obtained with marked carbon formation due to elementary decomposition.

It may be noted that the production rate of hexafiuoropropene as shown in this example is about a hundred times greater than that described in the reduced-pressure pyrolysis process U.S. 2,758,138.

When the reactor is larger in volume and smaller in the ratio of internal surface to internal volume, the purity of the product is enhanced.

.4 In Table I, and elsewhere in the examples herein 2-C F connotes perfluorobutene-2, and the term Selectivity in percent of the total conversion (carbon atom basis) connotes the percent of selected products which are formed, as indicated in the tables, and the percent value is determined by the equation Selectivity:

(number of mole of the selected product) number of mole of CF in the entire product TABLE I Exp. No 1 2 3 02F; Space Velocity 3, 060 3, 080 3. 040 Steam Concentration in mole percent 23.9 59. 2 79.8 Total Contact Time (in Sea, total gas N.T.P.) 0. 86 0. 48 O. -4 C211; Conversion in percent 63. 6 50. 5 42. 2 Selectivity in percent of the total conversion (carbon atom basis):

Cai o-u 0. 8 0. 2 0. 0

c-CaFs. 22. 6 33. 0 43. 9

2-C4Fa. 2. 4 2. 3 1. 6

i-Ctlis- 13. 5 8. 4 3. 0

Residue" 4. 2 1. 2 Trace Fluorine dissociated 5. 7 5. 9 6. 2 C 1 Space-Time-Yield (mole/L/hr.) 29. 1 22. 4 17. 2

Example II Using the same reaction procedure as stated in Example I except for the reactant species employed, octafiuorocyclobutane (c-C F was pyrolyzed at 850 C. at a space velocity of 1850 moles/liter/hour under atmospheric pressure in the absence or presence of water vapor.

The effect of the presence of 63.7 mole percent of superheated steam on the reaction results is shown in Table II which clearly indicates that the formation of by-products, especially that of octafluoroisobutylene (i-C F is markedly suppressed without any appreciable decrease in the c-C F conversion.

TABLE II Steam Concentration in mole percent 0 63. 7

Contact time in sec., total gas (N.T.P.) 1. 94 (1. 8 e-Orlia conversion in percent 69. 5 67. 5 Selectivity in percent of the total conversion (carbon om basis)- Example III Following the same reaction procedure as described in Example I, a mixture consisting of 53 mole percent of tetrafluoroethylene and 47 mole percent of octafluorocyclobutane was pyrolyzed at 850 C. and at a space velocity of 2700 (mole/l./hr.) under atmospheric pressure. In Table III the results of the reaction in the absence of water vapor and in the presence of 62 percent (based on the moles of the total gas mixture) of water vapor are compared. As can be seen from the Table HI, the presence of steam in the reaction phase enhances the yield and production rate of hexafluoropropene as well as the selective conversion of octafluorocyclobutane with a remarkable suppression of by-product formation, especially of octafluoroisobutylene.

TABLE III Steam Concentration in mole percent l. 0

Contact time in sea, total gas (N.T.P.) Conversion:

C:F4 0-C4Fs Selectivity in percent of the total conversion (carbon atrm basis):

Fluorine dissociated C Ft Space-Time-Yield (mole/lJhr.)

N N 0' sw s s s r OOMDJHriiDHO t- @wa-s m rp @Hb- HOaQuhlO Example IV Following the same reaction procedure as described in Example I, a series of mixtures of tetrafluoroethylene and octafluorocyclobutane of varying composition were pyrolyzed at 800:2 C. and at a space velocity of from 7500 to 8500 moles/liter/hour under ordinary pressure in the presence of 66-68 mol percent of water vapor. By comparing the reaction results, as shown in Table IV, a certain synergistic effect due to the presence of the two reactants, i.e., tetrafluoroethylene and octafluorocyclobutane, may be noted in that the total conversion (carbon atom basis) of the reactants as well as the selectivity and space-time-yield of the hexafiuoropropene product are noticeably enhanced as the mole ratio of tetrafiuoroethylene to octafluorocyclobutane is varied from 1:1 to 3:1.

TABLE IV Composition in mole percent:

Steam Concentration in mole percent 57. 9 Contact Time in See, total gas (N.T.P.). 0.50 Conversion in mole percent:

CzF-i 46. 6 C-CaFs Conversion in percent (carbon atom basis). 46. 6 Selectivity in percent of the total Conversion (carbon atom basis):

Fluorine dissociated C 11 Space-Time-Yield (mole/l./hr.)

presence of water vapor under substantially adiabatic conditions in a reaction zone, the pyrolysis temperature being generated mainly by simultaneously mixing an in troducing the preheated vapor of said reaction component and the superheated water vapor into said reaction zone, and said water vapor being present as 50 to 95 mole percent of the total gaseous mixture.

2. A process for preparing hexafluoropropene Which comprises pyrolyzing a mixture consisting of tetrafluoroethylene and from 10 to mole percent of octafluorocyclobutane in the presence of water vapor under Substantially adiabatic conditions in a reaction zone at a temperature of about from 700 C. to 900 C., the pyrolysis temperature being generated mainly by simultaneously mixing and introducing the preheated vapor of said mixture and the superheated water vapor into said reaction zone, and said water vapor being present as 50 to mole percent of the total gaseous mixture.

3. A process as in claim 1 in which octafluorocyclobutane is the reactant species being pyrolyzed.

4. A process as in claim 3 in which the pyrolysis reaction is conducted at 850 C.

5. A process as in claim 4 in which the water vapor content of the total gaseous mixture is about 64 mol percent.

6. A process as in claim 2 in which the mixture being pyrolyzed consists of about one to three mols of tetrafluoroethylene per mol of octafiuorocyclobutane.

7. A process as in claim 6 in which the pyrolysis reaction is conducted at about 800 to 85 0 C.

8. A process as in claim 7 in which the water vapor content of the total gaseous mixture is about 62 to 68 mol percent.

References Cited UNITED STATES PATENTS 2,551,572 5/1961 Dowing et al. 260-653 2,758,138 8/1956 Nelson 260653.3 2,970,176 1/1961 Teneyck et a1 260653.3 3,308,174 3/1967 Edwards et al. 260--653.5

FOREIGN PATENTS 904,022 8/ 1962 Great Britain.

OTHER REFERENCES Atkinson et al., J. Chem. Soc. 1953, 2082-2087, copy in patent oifice sci. library.

DANIEL D. HORWITZ, Primary Examiner.

US. Cl. X.R. 

